Two isolates of type C viruses were obtained from Mus cervicolor. One of these isolates is immunologically related to the woolly monkey-gibbon ape group of infectious type C viruses. The other isolate shares antigenic determinants with type C virus on M. musculus. In addition, a new class of endogenous retrovirus was isolated from Mus cervicolor and Mus caroli. These viruses are morphologically distinct from all other retroviral classes and are unrelated by immunologic and nucleic acid homology to all murine type B and C viruses as well as members of other retroviral groups. BIBLIOGRAPHIC REFERENCES: Callahan, R., Lieber, M.M., Todaro, G.J., Graves, D.C. and Ferrer, J.F.: Bovine leukemia virus genes in the DNA of leukemic cattle. Science 192: 1005-1007, 1976. Callahan, R., Benveniste, R.E., Sherr, C.J., Schidlovsky, G. and Todaro, G.J.: A new class of genetically transmitted retrovirus isolated from Mus cervicolor. Proc. Natl. Acad. Sci. USA 73: 3579-3583, 1976.